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Prosecutors will seek the death penalty against a San Diego man accused in the mysterious slayings of three victims that began with a shooting on Christmas Eve 2013 in the parking lot of Westfield Mission Valley mall.

Carlo Mercado, 30, appeared in court Thursday and pleaded not guilty in the killings of brothers Salvatore “Sal” Belvedere, 22, and Gianni Belvedere, 24, and Gianni’s fiancée, Ilona Flint, 22.

Mercado, staring straight ahead, appeared somber and was soft-spoken in court, only replying “yes” and “yes sir” when a judge spoke to him. He’s slated to appear in court once again on Mar. 17 for a status conference.

Deputy District Attorney Brian Erickson said the District Attorney’s office will seek the death penalty in this case, which has been many twists and turns over the past two years.

Mission Valley Mall Christmas Eve Shooting: Images

Erickson said it does not appear there was any connection whatsoever between Mercado and the victims, and said the slayings appear random, or perhaps the result of a “road rage” incident.

He said it does not appear the killings were a hit, as has been speculated with this case in the past.

“There are no facts that would support anything that indicates that this was some sort of professional hit,” he added.

Still, the motive for the triple killings remains shrouded in mystery.

“It appears to be a chance, road rage-type incident where Mr. Mercado shot one of the victims and then came back and shot the other two,” Erickson explained. “It appears to be a random act that was taken upon these people, for no reason whatsoever.”

Erickson said that, because this is a death penalty case, Mercado’s trial may take longer to get moving. He said a trial date has not yet been set, but said his goal is to get the case tried within this year.

Triple Homicide Suspect Found Mentally Competent

A judge ruled Tuesday a man accused of killing three people on Christmas Eve two years ago is competent to stand trial for murder. NBC 7’s Bridget Naso has the story.

(Published Tuesday, Dec. 15, 2015)

Mercado’s defense attorney, Gary Gibson, said he is disappointed with the DA’s decision to pursue the death penalty in this case given his client's history of mental health concerns.

Gibson said Mercado is a “deeply damaged individual with significant mental health issues." Up until this case, Gibsons said Mercado had lived a “blameless life” and had been mentally healthy.

Erickson argued that Mercado’s mental health issues are directly linked to this case and include depression “based on his situation."

On Dec. 24, 2013, Flint and Sal were found critically shot inside their car parked outside a Macy’s department store at Westfield Mission Valley mall in San Diego’s Mission Valley area. Flint, who called 911 to report the shooting and their location, died at the scene. Sal was hospitalized and died a few days later.

Flint’s fiancé and Sal’s brother, Gianni, went missing around the same time of the Christmas Eve killings. On Jan. 17, 2014, police found Gianni’s badly decomposed body stuffed into the trunk of his own car parked at a shopping center in Riverside, California, more than 100 miles away from San Diego. He, too, had been shot to death.

For six months, police reported no breaks in the baffling triple homicide case.

On June 20, 2014, the San Diego Police Department confirmed officers had arrested Mercado as the suspect in the three slayings. Mercado pleaded not guilty to three counts of first-degree murder.

At a pretrial in early September 2014, DNA evidence emerged linking Mercado to Gianni’s car and the bloody Riverside crime scene, while ballistics evidence linked a gun registered in Mercado’s name to the deadly shootings of Flint, Sal and Gianni. Prosecutors also presented evidence found on Mercado's phone and computers.

Also in early September 2014, search warrants obtained by NBC 7 revealed the exhaustive investigation into the triple homicide case, but no clear motive for the killings.

More Details Emerge in Triple Homicide Mystery

The pretrial continued on Sept. 3, 2014, for Carlo Mercado, the San Diego man accused of killing brothers Salvatore and Gianni Belvedere and Ilona Flint. As additional witnesses hit the stand, more details emerged on the triple homicide mystery. NBC 7's Elena Gomez reports.

(Published Wednesday, Sept. 3, 2014)

In December 2014 the families of the three victims filed a wrongful death lawsuit against Westfield, LLC, accusing the Mission Valley mall of negligence in the deaths of Flint and the Belvedere brothers, claiming the mall failed to provide sufficient lighting and monitoring security cameras in the area to keep patrons safe. That lawsuit also listed Mercado as a defendant, accusing him of malice and oppression in the killings.

On Nov. 3, 2014, a San Diego judge ruled Mercado was not competent to stand trial in the triple killings, and ordered he be treated at Patton State Hospital for three years until he was found competent to assist in his own defense.

That ruling came after reports submitted by two psychiatrists and one psychologist diagnosed Mercado as schizophrenic, psychotic and suffering from catatonic depression, Mercado’s attorney said at the time.

In September 2015, Mercado was returned to San Diego Central Jail after evaluators from Patton State Hospital found him competent to stand trial. The defense then requested a competency trial for Mercado.